The invention relates to a room temperature curing binder for refractory or foundry applications. The binder incorporates lightburned magnesium oxide particles.
The control of the ambient temperature curing of binder compositions is useful in the foundry and refractory arts. In foundry and refractory applications, the binder is mixed with sand or other refractory material and various shapes are formed with the help of pattern equipment. Room temperature or ambient temperature hardening systems, used in foundry and refractory applications depend on their ability to uniformly coat sand or other refractory grains which then cure into strong, rigid shapes at ambient conditions.
One room temperature cure process, known as ester cure, uses a highly alkaline phenolic resole resin with a pH greater than 11 and an alkali to phenol molar ratio of 0.2/1 to 1.2/1. Generally, the binder components are mixed into sand in either a batch or a continuous process and the mixed sand is discharged into a pattern. The sand begins to cure or harden immediately and it is essential to sufficiently pack the sand to achieve optimum bonding strength. If the sand cannot be sufficiently packed prior to curing there is a diminished bonding strength. The usefulness of a binder is related to the amount of time available to sufficiently work the binder into a pattern prior to curing of the binder.
Compositions for retarding the ambient temperature hardening of a phenolic resole resin alone or with an aggregate when such resin is contacted with a nitroalkane and a hardening agent such as lightburned magnesium oxide have been used. The pH of the phenolic resole resin used in that application varied over a broad range from about 4.5 to 13. However, hardening takes place at a pH above 7, i.e., in the alkaline range such as that above 7.5. When the hardener is lightburned magnesium oxide alone or together with an ester functional hardening agent, the pH of the resin can be below 7 such as between a pH of 4.5 and 7, but a sufficient amount of the lightburned magnesium oxide needs to be present to both neutralize the acidity and to provide sufficient magnesium oxide for the crosslinking and hardening of the resin.
Ambient temperature hardening of compositions containing magnesia aggregate and a curable, liquid phenolic resin, either alone or together with an ester function hardening agent, has been accelerated by the use of additives such as those which supply: acetate; adipate; 1,2,4-benzenetricarboxylate (trimellitate); formate; glycolate; lactate; nitrate; succinate; sulfamate; phenolsulfonate; or toluenesulfonate anions to the composition or compounds which supply acetylacetone (2,4-pentanedione); 2-nitrophenol; 4-nitrophenol; or salicylaldehyde to the composition. This curing method has been used with resole resins having a molar ratio of phenol-to-formaldehyde of between about 1:0.9 and 1:3. However, the phenol-to-formaldehyde ratios on the lower end of this range do not produce high strength binders in a short period of time. A novolac resin could also be used as a liquid solution if used alone as the phenolic resin or as a liquid or solid when used together with a resole solution.
Lightburned magnesium oxide products having different surface areas can be obtained from various sources such as the Martin Marietta Magnesia Specialties Company, Baltimore, Md., under the designator of MAGCHEM Magnesium Oxide Products. Lightburned magnesium oxides with the higher surface areas are more active and provide shorter times for gelation and hardening. Reactivity and surface area of magnesium oxide (magnesia) differ greatly depending on the procedure used for manufacture of the magnesia. Thus, lightburned magnesia has a surface area of about 10 to 200 or more square meters per gram. Hardburned magnesia and deadburned magnesia have a surface area of about one or less than one square meter per gram.
For use in refractory compositions, the magnesia grain has been crushed and sized in various fractions. Commonly used grain sizes of deadburned or hardburned grades of magnesia have been used for room temperature hardening, meaning the hardening of binder-aggregate compositions took place at temperatures of about 60xc2x0 F. to 90xc2x0 F.
Known binder-aggregate compositions produced by combining a curable resin binder, magnesia aggregate, and accelerator have additionally comprised a number of optional modifiers or additives including: non-reactive solvents; silanes; hexamethylenetetraamine; clays; graphite; iron oxide; carbon pitch; silicon dioxide; metal powders such as aluminum, magnesium, and silicon; surfactants; dispersants; air detraining agents; and mixtures thereof.
There is a need for a refractory mixture that cures at room temperature that provides adequate work time and then cures quickly and has sufficient compressive strength. There is further a need for such a refractory composition that provides a lower free phenol content and can be produced at a lower cost due to a decrease in the required resin concentration.
The present invention provides refractory compositions that minimize resin content. The advantages related to decreased resin concentration in the refractory mixtures are two-fold. First, the product cost is reduced, and secondly, there is a reduction in emissions associated with the resin. The compositions are workable and exhibit high compressive strengths in a short period of time. Additionally, the refractory mixtures of the present invention have a low free phenol content and require a reduced resin content as compared to compositions of the prior art. The refractory mixtures provided in the invention further reduce the cost to produce useful articles by eliminating the requirement for ester curing in addition to requiring reduced amounts of resin to obtain adequate compressive strength.
The present invention is directed to a composition including a liquid resole having a mole ratio of phenol to formaldehyde ranging from about 1:2.0 to about 1:2.4; an aggregate; and a magnesium hardening agent.